This invention relates to catalysts useful in polymerizing .alpha.-olefins. In particular, it relates to the polymerization of ethylene using transition metal catalysts with bidentate ligands containing pyridine or quinoline moieties.
Until recently, polyolefins have been made primarily using conventional Ziegler catalyst systems. A Ziegler catalyst typically consists of a transition metal-containing compound and one or more organometallic compounds. For example, polyethylene has been made using Ziegler catalysts such as titanium trichloride and diethylaluminum chloride, or a mixture of titanium tetrachloride, vanadium oxytrichloride, and triethylaluminum. These catalysts are inexpensive but they have low activity and therefore must be used at high concentrations. The catalyst residue in the polymers produces a yellow or grey color and poor ultraviolet and long term stability, and chloride-containing residues can cause corrosion in polymer processing equipment. It is therefore sometimes necessary to either remove catalyst residues from the polymer or add neutralizing agents and stabilizers to the polymer to overcome the deleterious effects of the residues and this adds to production costs. Furthermore, Ziegler catalysts produce polymers having a broad molecular weight distribution, which is undesirable for some applications, such as injection molding. They are also poor at incorporating .alpha.-olefin co-monomers, making it difficult to control polymer density. Large quantities of excess co-monomer may be required to achieve a certain density and many higher .alpha.-olefins, such as 1-octene, can be incorporated at only very low levels, if at all.
Although substantial improvements in Ziegler catalyst systems have occurred since their discovery, these catalysts are now being replaced with recently discovered metallocene catalyst systems. A metallocene catalyst typically consists of a transition metal compound that has one or more cyclopentadienyl ring ligands. Metallocenes have low activities when used with organometallic compounds, such as aluminum alkyls, which are used with traditional Ziegler catalysts, but very high activities when used with aluminoxanes as cocatalysts. The activities are generally so high that catalyst residues need not be removed from the polymer. Furthermore, they produce polymers with high molecular weights and narrow molecular weight distributions. They also incorporate .alpha.-olefin co-monomers well.
However, at higher temperatures metallocene catalysts tend to produce lower molecular weight polymers. Thus, they are useful for gas phase and slurry polymerizations of ethylene, which are conducted at about 80.degree. C. to about 95.degree. C., but in general they do not work well as temperatures are increased. The polymerization of ethylene in solution at higher temperatures is desirable because it allows great flexibility for producing polymers over a wide range of molecular weights and densities as well as the use of a large variety of different co-monomers. Solution polymerization permits the production of polymers that are useful in many different applications. For example, both high molecular weight, high density polyethylene (PE) film useful as a barrier film for food packaging and low density ethylene co-polymers with good toughness and high impact strength can be made.